This invention relates to a corrosion monitoring probe.
It is desirable to be able to measure the rate at which a corrosive material corrodes equipment in which such a material is contained. For example, a chemical plant may be corroded by the reactants and/or by the product of reactions taking place in the plant and it is clearly desirable, by means of a suitable monitoring means, to be able to detect whether or not corrosion is taking place, and if so the rate of corrosion, so that remedial action may be taken before the extent of the corrosion becomes so great that failure of the plant occurs, possibly with catastrophic consequences. This is particularly important at regions of the plant which may be particularly prone to corrosion.
Corrosion monitoring probes are known which fulfil this function. For example, a probe is known which comprises a body member, generally of tubular shape, and a pair of electrodes which project from the body and which are sealed to the body at the end thereof by means of a seal of an electrically insulating material which may be, for example, a ceramic seal or an organic resin seal, e.g. an epoxy resin. The body of the probe may be inserted into the plant which contains the corrosive material with the electrodes projecting into the corrosive material. In order to measure the rate of corrosion of the electrodes a small potential (e.g. 20 mv) is applied across the electrodes to cause a small current to flow through the corrosive material. This current varies with the corrosion of the electrodes and measurement of the current may be used to obtain a measure of the rate of corrosion of the material of the electrodes. The probe may contain three electrodes, one being a reference electrode. This probe may only be used to measure the corrosion caused by electrically-conducting liquids.
The probe comprises a body member and a metallic probe element which is sealed to the body by means of a seal of an electrically insulating material which may be, for example a ceramic seal or an organic resin seal, e.g. an epoxy resin seal. The upper part of the probe element in the form of a continuous wire connecting the two arms of the probe element, which in use is corroded, projects from the seal and is protected from mechanical damage by an open-ended shield. The body may house a metallic check element and a metallic reference element which are protected from the corrosive material by means of the seal. The body may be equipped with means for securing it to the equipment which contains the corrosive material. For example, where the probe is to be used to monitor the corrosion in a chemical plant the body may be equipped with an external screw-thread by means of which it may be secured to the plant in a corresponding screw-threaded aperture in the plant, or it may be secured in an aperture in the plant by means of a gland.
In use, the probe element is corroded by the corrosive material and as the cross-sectional area of the element is decreased due to the corrosion the electrical resistance of the element increases. A measure of the corrosion taking place may be determined by measuring the resistance of the probe element at intervals of time and noting the change in resistance.
If the resistance of the probe element is measured directly the data obtained will be affected by the temperature of the element as well as by its cross-sectional area. In order to eliminate variations in the measured resistance due to variations in temperature the corrosion monitoring probe preferably also includes a reference element and optionally a check element. In use the resistance of the probe element is compared with the resistance of the reference element, which will be at the same temperature as the probe element, using a Wheatstone bridge arrangement. Before taking a measurement the resistance of the reference element may be checked against that of a check element to ensure that the resistance of the former element has not changed.
Corrosion monitoring probes of the aforementioned types suffer from a disadvantage in that during use the seal may be damaged, for example, by mechanical action and/or by chemical action of the corrosive material on the seal. The extent of the damage may be such that the corrosive material is allowed to pass the seal and penetrate into the body of the probe. The result of penetration of corrosive material into the body of the probe may be that incorrect readings of the current passing between the electrodes, or incorrect readings of the electrical resistance of the metallic probe element, may be obtained. Incorrect measures of the rate of corrosion will thus be obtained. In the case where the body of the probe contains a reference element, the corrosive material which penetrates into the body of the probe may attack the reference element and change its resistance so that when in use the resistance of the probe element is compared with that of a corrosion-affected reference element an incorrect measurement of the resistance of the probe element will be obtained.
We now provide a corrosion monitoring probe in which the possibility of corrosive material penetrating the body of the probe is at least much reduced, and may even be substantially eliminated.
The present invention provides a corrosion monitoring probe comprising a body member, a seal within the body member, and a metallic probe element which is contained in part within the body member and which in part projects beyond the seal, at least that part of the probe element which projects beyond the seal being corrodable, in which the seal is in the form of at least one gasket made of an electrically insulating compressible material and in which the probe is provided with means for compressing the gasket(s) to cause the gasket(s) to form a seal between the probe element and the body member.
The metallic probe element may be of the type comprising two or more electrodes in which corrosion of the probe element is determined by measuring the change which occurs with corrosion of the electrodes in the current which passes between the electrodes when a given small potential difference is applied across the electrodes.
Alternatively, the metallic probe element may be of the type comprising a continuous element in which corrosion of the probe element is determined by measuring the change in electrical resistance of the probe element which occurs with corrosion of the element.
Use of the former type is limited to corrosive materials which are liquid and which are electrically-conducting. The latter type may be used with such corrosive electrically-conducting liquid materials and in addition may be used with non-conducting corrosive liquid materials, for example, hydrocarbons, and with corrosive gaseous materials.
The probes may be fitted with means for connecting the probe element to suitable electrical equipment, in the case of the probe element comprising at least two electrodes to means for generating a potential difference across the electrodes and to means for measuring the resultant current, and in the case of the probe element comprising a continuous element to means for measuring the electrical resistance of the element. In the latter case the body of the corrosion monitoring probe may contain a reference element the resistance of which may be compared with that of the probe element using a Wheatstone bridge arrangement as hereinbefore described. The body of the probe may also contain a check element.
The probe element may be made of any desired metal the corrosion of which it is desired to determine. However, where the corrosion monitoring probe is to be used to measure corrosion in a chemical plant the probe element will generally be made of the same metal as that from which the plant is constructed, or at least of the same metal as that part of the plant where the probe is installed. The probe element may for example be made of mild steel, stainless steel or titanium.
The body member of the corrosion monitoring probe, which in general will be metallic and which is preferably made of a material which is not significantly corroded by the corrosive material, may be of tubular shape and suitably includes an annular lip on which an annular gasket may bear and against which a gasket may be compressed in order to effect a seal between the probe element and the body thereby preventing ingress of corrosive material into the body of the probe.
The means for compressing the gasket(s) may be an annular sleeve or sleeves positioned within the body member. The sleeve may be externally screw-threaded and it may be attached to the body member via a corresponding internal screw-thread on the body. The sleeve may bear directly, or indirectly, on the gasket(s), tightening of the sleeve causing the gasket(s) to be compressed thereby effecting a seal.
By a gasket we mean a packing member of any desired shape which is capable of being compressed. The gasket may be generally flat, e.g. in the form of a flat disc, in the form of a sleeve, or in the form of a ring, e.g. a so-called O-ring. The corrosion monitoring probe may contain more than one gasket and the number of gaskets used, and their precise type, will depend on the precise configuration of the body member and of the probe element of the corrosion monitoring probe. For example, in the probe a gasket may be compressed and caused to bear on both the body member and the probe element thereby effecting a seal. Alternatively, the probe may contain a plurality of gaskets and at least one non-compressible member between the body member and the probe element with at least one gasket being compressed and bearing on the probe element and on the non-compressible member or members and at least one gasket being compressed and bearing on the body member and the non-compressible member or members thereby effecting a seal.
The material of the gasket(s), which will be an electrically insulating material, should be selected bearing in mind the nature of the corrosive material with which the probe and gasket(s) come into contact. Clearly, a material will be selected for the gasket(s) which is resistant to corrosion by the corrosive material. Materials from which the gasket(s) may be constructed include, polytetrafluoroethylene (PTFE), natural rubber, a synthetic rubber, e.g. neoprene rubber, and compressed asbestos fibre (CAF).
A particular embodiment of a corrosion monitoring probe of the invention comprises:
(a) a tubular body member having an annular lip at its upper end, PA1 (b) a probe element comprising at least two metallic discs each disc having a pin above and below the plane of the disc and a channel or channels through the disc, the discs being positioned one on top of the other so that the pins of a particular disc pass through the channels in the adjacent disc or discs, PA1 (c) a seal comprising a gasket or gaskets in the form of channelled disc(s) of an electrically insulating material positioned between adjacent metallic discs, and an annular gasket of an electrically insulating material positioned between the upper metallic disc and the lip of the body member, and PA1 (d) means for compressing the channelled disc(s) and the annular gasket so as to effect a seal.
At least part of the pins above the planes of the discs will be of a corrodable metal. If desired the pins and discs may be made in whole of a corrodable metal.
The means for compressing the seal may be a screw-threaded sleeve mounted on an internal screw thread on the body member. The screw-threaded sleeve should be insulated electrically from the discs of the probe element, e.g. by means of a disc of insulating material positioned below the bottom disc of the probe element.
The pins in the upper faces of the metallic discs may form the electrodes of a corrosion monitoring probe. Alternatively, the pins may be connected to form a corrodable continuous metallic element.
The probe element may comprise two metallic discs each disc having a pin above and below the plane of the disc and each disc having a channel through the plane of the disc to accommodate a pin of the other disc when the probe element is assembled in the corrosion monitoring probe. Thus, the pin on the lower face of the upper disc passes through the channel in the lower disc and the pin on the upper face of the lower disc passes through the channel in the upper disc.
The probe element may comprise three metallic discs each disc having a pin above and below the plane of the disc and each disc having two channels through the plane of the disc each channel accommodating a pin of the other discs when the probe element is assembled. The holes and pin in each disc will be symmetrically disposed on the face of each disc. The three pins in the probe element may function as corrodable electrodes in the corrosion monitoring probe.
In a further embodiment the probe element may comprise four metallic discs each disc having a pin above and below the plane of the disc and three channels through the plane of the disc each channel accommodating a pin of the other discs when the probe element is assembled. The holes and pin in each disc will be symmetrically disposed on the face of each disc. The four pins in the probe element may function as two pairs of corrodable electrodes. Alternatively, pairs of pins may be connected to form corrodable continuous metallic elements, or one pair of pins may serve as corrodable electrodes and one pair may be connected to form a corrodable continuous metallic element.
The pins may be provided with means for connection to suitable electrical equipment.
The seal may also include a sleeve of an electrically insulating material positioned in the annular space between the assembled metallic discs and discs of insulating material and the body member. Compression of the insulating discs causes them to bear on the sleeve and effect a seal with the body member.
It is the primary object of the present invention to provide an improved corrosion-monitoring probe.